Images on electronic displays are derived from a two-dimensional array of pixels, each of which represents one small element of the image. The resulting image is presented to the observer in a 1:1 size in direct-view displays, while projection displays magnify the image size, using an optical lens system. In black-and-white displays, each pixel displays one of two colors, black or white; in a gray-tone display, pixels can produce a specified number of gray tones between black and white. Since colors can be formed by combining primary colors red (R), blue (B) and green (G) light, in specified ratios, electronic color displays use primary-color elements in each pixel, in order to form a desired image via additive color mixing. In order to show still images, pixels can carry the same information all of the time; for moving images, the content of each pixel must be redefined periodically. Depending on the application, full-motion images are usually required to be redrawn 30 to 75 times per second.
Pixels can be accessed by using several techniques, including scan-, grid-, shift-, matrix- and direct-addressing. If, for example, the display carries an array of N.times.M pixels, and it has to be redrawn n times each second, the data sent to each pixel must be provided in 1/(n*N*M) seconds and then held constant for (N*M-1)/(n*N*M) seconds, as other pixels are being defined. In the current American television (TV) standard (NTSC), each frame has about 250,000 pixels, with an aspect ratio of 4.times.3, which are refreshed at the rate of 30 frames/second. One of the new picture formats proposed to the Federal Communications Commission (FCC) for American high-definition television (HDTV) has an aspect ratio of 16.times.9 and a refresh rate of 60 frames/second. Pixels are arranged into 1280 horizontal and 720 vertical lines or, alternatively, 1920 horizontal and 1080 vertical lines (I. Gorog, "Displays for HDTV: Direct View CRTs and Projection Systems", Proceedings of the IEEE, vol. 82, no. 4, pp. 520-536, 1994). The typical, low-resolution computer display (VGA) has 480 rows of 640 pixels, or, a total of 307,200 pixels at a refresh rate of 72 frames/second.
Electronic displays can be implemented by using a multitude of different technologies, including, for example, the cathode-ray tube (CRT), electroluminescent displays (ELDs), light-emitting diode displays (LEDs) and liquid-crystal displays (LCDs). While a CRT display has a depth comparable to the height of the screen, ELDs and LCDs belong to that class of flat-panel displays (FPDs), the dimension of which, in their direction perpendicular to the image plane, is much smaller than that of the CRT. With the CRT, either one (gray-tone) or three (color) electron beams scan along horizontal lines in order to access each pixel. All color signals are thus carried to the pixels via the electron beam flux. FPDs (such as the LCDs) use matrix-addressing, in which each pixel is accessed via row and column lines. The column lines usually carry the color signals, while row lines are used for control signals. The pixel at the cross-point of a specific row and column line can be selected via passive or active techniques. In the passive case, the non-linearity of the pixel's element is used for the selection. For example, in LCDs the non-linearity of the liquid-crystal material is used. Active, matrix-addressed LCDs (AMLCDs), on the other hand, require a device (e.g., a transistor) for the selection of the pixel. In active matrix-addressing, a row of pixels is usually selected at once by placing a specific, control signal on the row line (usually a voltage on a transistor's gate electrode). Pixel color data is then made available via column lines to each of the pixel elements in the selected row (usually a voltage on a transistor's drain). An entire row of pixels can be accessed in parallel in active matrix-addressing. Coupling between pixels and row and column lines is one of the disadvantages of matrix-addressing.
The size of an electronic display is usually specified by the length of the diagonal of the pixel array. Computer displays generally have sizes of between 10" and 21"; home television displays generally have sizes of between 19" and 31". Large public displays (e.g., used in sports arenas) generally feature sizes that range between 200" and 700".
The resolution of the image on an electronic display is determined by the pitch of the pixels, i.e., the smaller the pixel pitch, the finer the details that can be displayed. Typical computer displays have pixel pitches on the order of 0.25 to 0.3 mm, and they can be viewed from distances as close as 30 cm without the human eye having to resolve the mosaic structure of the pixels. Large-screen, public displays have pixel pitches as large as 30 mm [see, e.g., Panasonic Astrovision, AZ-3000 Series High Density Fluorescent Displays, Panasonic Corporation, Japan, 1995]. Viewing distances of at least 10 meters are required for such displays.
A duty cycle is defined as the time spent for turning on individual pixels or a row of pixels. With a CRT, each pixel is accessed individually and sequentially by sweeping the electron beam. Thus, for example, in a VGA display with N.times.M=640.times.480 and n=72 Hz, the dwell time of the electron beam on each pixel is 46 ns. By definition this equals the duty cycle of this CRT. In an FPD VGA display with the same frame rate, the dwell time is 640 times longer or 29 .mu.s, due to parallel matrix-addressing.
The brightness of an image on an electronic display is characterized by using the photometric quantity of luminance measured in candelas per unit area (cd/m2=1 nit). The luminous efficiency is used to describe how much light the display produces per the amount of electrical energy provided to the display. LCDs operate with highly efficient backlights (such as fluorescent lamps) with a luminous efficiency as high as 55 lm/W and a typical light transmittance of about 4%. This gives a typical luminous efficiency of 2.2 lm/W for AMLCDS, which exceeds the performance of all other display technologies. The brightness of LCDs can be increased by simply turning up the intensity of the backlight.
The contrast in a display is another important attribute. It describes the achievable light intensity modulation in the image between the brightest and dimmest pixels. An image having a greater contrast is more sparkling in appearance. The best AMLCDs achieve contrast ratios as large as 100:1. Ambient illumination affects the contrast of the displayed image. The component of the ambient illumination that is reflected from the display's surfaces will be added to the emitted intensity of the image to be displayed. The higher the contrast, the more tolerant the display is to ambient light. Of all displays, AMLCDs have the highest tolerance to ambient light, because of the presence of polarizers, and the ability of AMLCDs to independently adjust the intensity of the backlight.
The viewing characteristics of electronic displays are specified by the viewing distance and viewing angle ranges. The minimum viewing distance is related to the pixel pitch via the resolution ability of the observer's retina. Displayed images are usually best viewed at normal incidence. Maximum horizontal and vertical viewing angles away from the normal are determined by the type of the display, and the layout and the optical design of the pixels. Viewing angle ranges of .+-.30.degree. horizontal and .+-.15.degree. vertical are average for typical AMLCD displays.
Full-color displays are expected to be able to display 256 (8-bit) shades of each of the highly saturated primary colors red, blue and green. This results in a total of 256.sup.3 or 16,777,216 colors that (in principle) can be displayed. Full-color capability has been available on CRTs for quite some time via the selection of the R, B and G phosphor materials, as well as the control of the electron beam. Full color was demonstrated for the first time with LCDs in 1993 by developing 8-bit data driver circuits [G. H. Henck Van Leeuven et al., "A Digital Column Driver IC for AMLCDs", Euro-Display, pp. 453-456, 1993; see also H. Okada, K. Tanaka, S. Tamai and S. Tanaka, "An 8-Bit Digital Data Driver for AMLCDs", Society for Information Display International Symposium Digest of Technical Papers, vol. XXV, pp. 347-350, 1994]. To date, several manufacturers have demonstrated full-color AMLCDs by using amorphous silicon (a-Si), thin-film transistors (TFT) as the switches. Saturated primary colors are defined by using a uniform "white" backlight in combination with three color filters. Driver electronics is used to provide an optimal linearization of the liquid-crystal response, in order to facilitate the additive mixing of colors.
Direct-view electronic displays with diagonals up to about 31" are usually manufactured in monolithic form, with the entire pixel array fabricated on a single continuous medium. The size of a commercial color CRT is limited by the deflection optics and the weight of the unit to about 35". Commercial, monolithic AMLCDs are currently limited to sizes less than 12" because of manufacturing yield and cost. Commercial, 16" AMLCD displays are in product development. AMLCD sizes of up to 21" have been demonstrated in research [M. Hijikigawa and H. Take, "Future Prospects of Large-Area Direct View LCDs", Society for Information Display International Symposium Digest of Technical Papers, vol. XXVI, pp. 147-149, 1995]. Very large electronic displays cannot be made in a monolithic fashion. Rather, each pixel is separately fabricated, and then the display array is assembled by accurately arranging pixels into rows and columns. The alignment process is difficult and cannot be made with high precision over large areas. As a consequence, the pixel pitch in large-screen displays usually is on the order of at least 30 mm.
Intermediate-sized electronic displays with pixel pitches from about 0.6 to 3 mm, can, in principle, be assembled from smaller monolithic pieces, with each carrying many pixels [see, e.g., N. Mazurek, T. Zammit, R. Blose and J. Bernkopf, "A 51-in Diagonal Tiled LCD VGA Monitor", Society for Information Display International Symposium Digest of Technical Papers, vol. 24, pp. 614-617, 1993]. These monolithic pieces are then arranged into a regular, tiled array to form the full display. In tiled displays, the pixel pitch on all tiles is, preferably, the same. Because of the small size of the tiles, this can be achieved with a tightly-controlled manufacturing process. The seams between adjacent tiles must be large enough to facilitate assembly. The seams will be visible to the human observer, unless the pixel spacing across the seam is the same as the pixel spacing on the tiles. This is very difficult to achieve. Consequently, to date, commercial-prototype, tiled displays have had visible seams between the tiles. The minimum achievable pixel pitch in tiled displays is, therefore, determined by the available assembly technology.